Make sure that you do not run Firefox in full screen mode (press F11 or Fn + F11 to toggle; Mac: command+Shift+F).
Press F10 or tap the Alt key to bring up the "Menu Bar" temporarily if the Menu Bar is hidden.
See also:
*https://support.mozilla.com/kb/Menu+bar+is+missing
*https://support.mozilla.com/kb/how-do-i-get-firefox-button

In Firefox 3.6 versions on Windows and Firefox 4 and later on Windows and Linux it is possible to hide the "Menu Bar" via "View > Toolbars" or via the right-click context menu of a toolbar.
Tap the Alt key or press the F10 key to bring up the menu bar.
Go to "View > Toolbars" or right-click the menu bar or empty space on the tab bar to select which toolbars to show or hide (click on an entry to toggle the state).
See also:
*https://support.mozilla.com/kb/Menu+bar+is+missing